The present invention relates to butterfly valves in general, and more particularly to a butterfly valve which is especially suited for use in dead-end service.
There are already known various constructions of butterfly valves which usually include a valve housing or body that bounds a passage, a disc-shaped valve member which is received in the passage and mounted for turning about an axis extending transversely of the passage, and an annular sealing element extending around the passage and sealing the gap between the valve body and the valve member in the closed position of the latter.
There have also been developed so called insert or cartridge-type butterfly valves in which the valve body is substantially annular and is inserted between the flanges of two consecutive pipe sections when in use. Examples of butterfly valves of this type are shown in U.S. Pat. Nos. 3,540,691; 3,656,714; 3,782,684; and 4,101,112. These patents show that it is known to construct the sealing element of two different materials, that is, a relatively rigid material which constitutes a support ring, and an elastically yieldable material in the form of a sealing ring which at least partially embeds the support ring. In each instance, however, the material of the sealing ring covers at least the inner peripheral surface of the support ring, and its two axial faces in their entirety. The reasons for this should be easy to appreciate. The portion which covers the inner peripheral surface of the support ring is in a sealing contact with the periphery of the disc shaped valve member around the entire periphery of the passage in the closed position of the valve member. On the other hand, the portions of the annular sealing ring which cover the axial end faces of the support ring provide for a sealing contact with the flanges of the consecutive pipe sections upon incorporation of the cartridge-type butterfly valve in a pipeline, thus preventing leakage externally and past the sealing element in the closed position of the valve member.
As advantageous as the construction and operation of these butterfly valves may be when they are interposed between two consecutive pipe sections, experience has shown that they suffer of a serious drawback when it is attempted to use them in dead-end service, that is, at the free end of a pipe or a similar tubular element. In this case, the axial forces acting on the sealing element and on the disc-shaped valve member in its closed position due to the pressure of the fluid thereon will be resisted only by the stem mounting the valve member in the passage. The stem will have to accept and withstand not only the forces resulting from the pressure of the fluid on the valve member itself, but also those which act on the sealing element in the axial direction and press the same against the stem. It was established that, under these circumstances, there is a pronounced danger that the stem could bend or, even if it does not, that the sealing element could be displaced by the pressure of the fluid thereon in the downstream direction to an extent sufficient for leakage to occur either outwardly past the sealing element, or through a leakage path coming into existance between the inner periphery of the sealing element and the outer periphery of the valve member in the thus axially displaced position of the sealing element. In the above-listed patents, this problem exists even though the sealing element is in contact with portions of the valve housing or body or other elements which have a reduced diameter with respect to the remainder of the passage, in that the elastically yieldable portions of the sealing element are in such contact.